Several types of phase shifters exist. For example, a line-switching-type phase shifter is known in which 50Ω-lines of different lengths are connected by two SPDTs (single-pole, double-throw) switches and the difference in the electrical lengths of the two lines obtained by switching is used as the amount of phase shift. There is also a filter-switching-type phase shifter in which a plurality of inductances or capacitances of a lumped constant and SPDT switches are combined and use is made of a phase difference that is the result of electrically switching between an LC-type low-pass filter (LPF) and high-pass filter (HPF). The switching portion of a phase shifter also is of various types, such as of the series-shunt type that makes joint use of series-connected FETs (field-effect transistors) and shunt-connected FETs, and resonance type (see Patent Document 1) in which inductors are connected in parallel with FETs.
In such phase shifters, a resonance-type switch of the kind shown in FIG. 11A is used widely in the SPDT switch circuit (see Patent Document 1). As shown in FIG. 11A, the resonance switch comprises a parallel-connected field-effect transistor (referred to simply as a “transistor” below) T10, which serves as a switching element, a resistor R10, and an inductor L10. When such a resonance switch is turned off, a parallel resonance circuit is formed by the capacitance of the transistor T11, when the transistor is OFF, and the inductor L10. In a desired frequency band, therefore, the resonance switch attains a high impedance and exhibits an excellent cut-off characteristic. In a case where pass/fail of the switching transistor T10 is inspected, however, the fact that the drain and source of the transistor T10 are connected directly by the inductor means that a direct current flows through the inductor L10, and DC measurement cannot be carried out.
On the other hand, Patent Document 2 discloses a switch circuit in which, in a process for inspecting a semiconductor device that includes a resonance switch circuit, it is made possible to measure the DC characteristic of the switching element in this circuit. As illustrated in FIG. 11B, this switch circuit includes a resistor R10, transistor T10 having first, second and third electrodes P1, P2, and P3, respectively, in which a high-frequency signal is passed or cut off between the first electrode P1 and second electrode P2 based upon the potential of the third electrode P3, and bias voltages at the first electrode P1 and second electrode P2 are approximately the same; and an inductor L10 and capacitor C10 connected in parallel with the transistor T10 at the first electrode P1 and second electrode P2 and connected in series with each other. In accordance with such a switch circuit, direct current is cut off by the capacitor C10, so that the DC characteristic of the transistor T10 can be measured.
[Patent Document 1] Japanese Patent Kokai Publication No. JP-P2006-19823A
[Patent Document 2] Japanese Patent Kokai Publication No. JP-P2005-117634A